[doc][dpapp] Initial bmv2 dpapp HLD (#606)

The document is a high level design doc of BMv2 data plane app. As a proof of concept, it describes the design of a data-plane app example, how it cooperates with [DASH pipeline BMv2](https://github.com/sonic-net/DASH/tree/main/dash-pipeline/bmv2) to implement DASH data plane. The app will be based on [VPP](https://fd.io/).

DPAPP is another packet processing engine running on CPUs. It adds extra capacities onto DASH capable ASIC:
- It serves the slow (exception) path of packet processing to implement complicated pipeline logic, which is hard to be done in ASIC, for example adding/updating/removing table entry inline.
- It optionally implements some DASH-SAI APIs, which are not proper/easy to be done upon ASIC SDK.

.wordlist.txt

@@ -100,6 +100,7 @@ configs
 Conntrack
 Containerlab
 CP
+CPUs
 CreatedHalfOpenFlow
 CreatedOtherFlow
 CreatedTcpFlow
@@ -120,6 +121,7 @@ CurrentUdpFlow
 customizable
 Cx
 cyberithub
+dashsai
 dashsubmodule
 DASHOrch
 dashorch
@@ -146,6 +148,8 @@ decap'ing
 decaps
 deliverables
 DemoDays
+dequeue
+dequeues
 designator
 DEST
 dest
@@ -171,6 +175,8 @@ dockerized
 DoS
 DotNet
 downcasting
+dpapp
+DPAPP
 DPDK
 DPU
 dpu
@@ -215,11 +221,14 @@ eni
 Eni
 ENIs
 ENI's
+enqueue
+enqueues
 entrypoints
 enum
 EPUs
 EPYC
 ethernet
+EtherType
 executables
 ExpressRoute
 extern
@@ -229,6 +238,7 @@ failovers
 fakesai
 FastPath
 fc
+FDio
 FEC
 ffe
 FINPackets
@@ -315,6 +325,7 @@ Jinja
 jitter
 journaled
 json
+Junhua
 keepalive
 keepalives
 Keysight
@@ -507,6 +518,7 @@ routable
 RPC
 RPCs
 RPF
+RSS
 RST
 RSTPackets
 RSTs
@@ -695,6 +707,9 @@ vport
 VPort
 vPORT
 VPorts
+vpp
+VPP
+vppctl
 VTEP
 VTEPs
 VxLAN
@@ -738,6 +753,7 @@ Gan
 Ze
 AppDBMemoryEstimation
 Ivantsiv
+Zhai
 Zhixiong
 Niu
 validonly